Robots Atlas>ROBOTS ATLAS
Robotics

PMSM

ActivePublished: 21 October 2026Updated: 21 October 2026Published
Key innovation
Combining a permanent-magnet rotor with sinusoidal back-EMF and vector (FOC) control delivers smooth, precise torque and high power density without a mechanical commutator.
Category
Robotics
Abstraction level
Building block
Operation level
Robot control
Use cases
Servo drives and robot actuatorsHumanoid robot joint drivesElectric-vehicle tractionCNC machine-tool spindles and axesHigh-efficiency industrial drives

How it works

The inverter-fed three-phase stator winding produces a rotating magnetic field. The rotor's permanent magnets lock onto this field and rotate synchronously with its frequency. The FOC controller measures rotor position (encoder, resolver or Hall sensors) and, via the Clarke and Park transforms, maps the phase currents into the rotating d-q reference frame, decoupling the torque-producing component (iq) from the flux-producing one (id). PI controllers hold the commanded currents while modulation (e.g. SVPWM) drives the inverter transistors. Keeping idโ‰ˆ0 maximizes torque per amp; forcing a negative id enables field weakening to extend the speed range above base speed.

Problem solved

It removes the mechanical commutator and brushes of DC motors, eliminating their wear and arcing, while its sinusoidal back-EMF and FOC drive reduce the torque ripple typical of simpler six-step (BLDC) commutation. It enables the precise, dynamic torque and speed control required by servo drives and robot actuators.

Components

Permanent-magnet rotorProduces the rotor's constant magnetic field

Rotor carrying permanent magnets (usually NdFeB or ferrite) mounted on the surface (SPMSM) or buried inside (IPMSM). The IPMSM variant provides additional reluctance torque.

Three-phase stator windingGenerates the rotating magnetic field

Stationary three-phase winding fed by sinusoidal currents whose field rotates at the supply frequency and pulls the rotor along.

Rotor position sensorProvides rotor angle for FOC control

Encoder, resolver or Hall sensors measuring rotor angle and speed; position accuracy is critical for the Park transform and torque quality. Sensorless estimation methods also exist.

Inverter with FOC controlRegulates phase currents in the d-q frame

Three-phase transistor inverter with PI current controllers and modulation (e.g. SVPWM), implementing field-oriented control that decouples torque and flux.

Implementation

Implementation pitfalls
Rotor position estimation errorHigh

FOC requires accurate rotor angle; encoder calibration error or phase offset degrades efficiency, increases torque ripple and can cause loss of synchronism.

Fix:Careful encoder alignment procedure, offset compensation and optionally sensorless estimation as a fallback.
Magnet demagnetizationMedium

High temperature or excessive field-weakening current (negative id) can irreversibly demagnetize the magnets, especially NdFeB grades with lower thermal resistance.

Fix:Select magnets with an adequate thermal grade, monitor temperature and limit d-axis current.

Evolution

1929
Park's transformation
Inflection point

R.H. Park publishes the two-axis (d-q) theory of synchronous machines, which became the mathematical foundation for the analysis and vector control of PMSMs.

1972
Field-oriented control (FOC)
Inflection point

Felix Blaschke formulates the principle of field orientation for AC machines, enabling decoupled torque and flux control of PMSMs.

FOC (concept)
1984
Neodymium (NdFeB) magnets
Inflection point

The development of high-energy-density Nd-Fe-B magnets (Sagawa/Sumitomo and, independently, General Motors) enabled compact, high-torque-density PMSMs.